1. Field of the Invention
The present invention relates generally to liquid transfer systems used for delivering chemical liquids to processing systems requiring high purity and an accurate liquid flow rate, and more particularly to pressurizable vessels used in liquid transfer systems to push liquid between a source and a processing system.
2. Description of the Prior Art
Liquid transfer systems typically provide for transfer of liquid chemical from one or more bulk sources to one or more end-use stations, via a series of conduits and controlled valves. An even flow rate through a liquid transfer system provides improved overall purity in the liquid delivered to the end use station which is very important in processes such as semiconductor manufacturing.
In many industrial processes, it is important to maintain process chemicals free of virtually all contaminants. For example, extremely high levels of purity are required for chemicals, such as acids and solvents, used in semi-conductor wafer production. As a result of these purity standards, precise controls are required in the delivery of the chemicals to such systems. High accuracy liquid transfer systems provide delivery of high purity chemicals at an even liquid flow rate.
The use of pumps is not desirable in high accuracy liquid transfer systems because pumps may cause a pulsed flow of liquid, that is flow occurring at different pressures and velocities through the system, due to the cyclical nature of diaphragm pumps and similar devices. This pulsed flow leads to problems of contamination of the transferred liquids. Therefore, liquid delivery systems which rely upon pumps to move the liquid are less desirable than systems which move liquid without utilizing pumps.
Liquid delivery systems that do not use pumps, generally utilize controlled gas pressure generated within a pressurizable liquid holding vessel to push liquid from the vessel to the end-use stations in a controllable manner. Such systems commonly include: an intake sub-system for transferring liquid from the bulk sources to the vessels; a dispensing sub-system for transferring the liquid from the vessels to one or more end-use stations; and a recirculation sub-system for transferring the liquid from the vessels to one of the other bulk sources during periods of low demand. The subsystems typically include: conduits and valving for transferring fluids including chemicals and compressed gas; pressure gauges and flow meters for monitoring system operation; and filtering units for filtering the liquid.
One common type of system utilizes two pressurizable liquid holding vessels to alternatively deliver liquid, wherein one vessel is filled while the second vessel is pressurized to dispense liquid therefrom. By alternately filling and dispensing liquid from two pressurizable vessels, a constant, controllable flow of liquid is obtained. However, the inputting of liquid into the vessels during the fill cycle can be problematic.
In one common type of liquid delivery system, a combination of vacuum and pressure is used to transfer liquid from the bulk source to the intermediate vessels. First, a vacuum pump is used to establish a vacuum in one of the vessels to draw liquid into the vessel. Once a vessel is filled, the vessel is then pressurized to motivate the liquid to an end-use station. However, the use of a constant vacuum, pumping, or significant pressure to move liquid from a supply tank to each vessel during the fill cycle can alter the delicate chemistry of some types of liquids, such as by removing volatile organic compounds from solvents or adding small bubbles into the liquids.
Schell (U.S. Pat. No. 5,832,948, issued Nov. 10, 1998) discloses a liquid transfer system including a liquid supply tank, and at least two pressurizable liquid holding vessels. The liquid holding vessels are placed beneath the supply tank, and a liquid supply line connects the supply tank to each vessel. The liquid supply line is operated as a siphon from the tank to each vessel, in order to move liquid from the tank to each vessel. Each vessel is alternately filled and pressurized to dispense liquid from the vessel, such that one vessel is being filled while the other is dispensing liquid, and a constant controllable liquid output flow is achieved. The system also includes a liquid recycling line to recycle or constantly move the liquid within the system to achieve thorough mixing, and an in-line filter to improve liquid purity. By placing the pressurizable vessels beneath the supply tank, a siphon effect can be utilized to transfer liquid from the supply tank to the vessels. Once a siphon effect has been established, there is no further need for pressure or continued vacuum effect to move the liquid from the supply tank to the vessels, thus improving the quality of the output liquid from the system.
In each of above described prior art liquid transfer systems, the intermediate vessels are provided with one or more level sensors to detect and identify the level of chemical in each of the vessels. These sensors may include high level sensors, high level redundant sensors, low level sensors, and low level redundant sensors. These sensors are commonly mounted outside of the vessels so as to avoid maintenance and contamination problems. The most commonly used level sensors are capacitive-type sensors, such as KGE model sensor available from EFECTOR of Camarillo, Calif. The level sensors are used to determine the amount of liquid in the intermediate vessels. The level sensors are used to determine the timing of liquid transfer operations including the filling and pressurizing of the intermediate vessels such that one vessel is being filled while the other is dispensing liquid, so that a constant controllable liquid output flow is achieved.
A major consideration in the design of liquid transfer systems, not previously mentioned, is the amount of space required for the liquid transfer apparatus. Typically, the components of a liquid transfer systems are enclosed within a cabinet along with semiconductor manufacturing apparatus. Therefore, it is desirable that each component of a system be as compact as possible.
The pressurizable vessels require a relatively large amount of space. The vessels are typically formed by tubular structures being closed on each end and formed a longitudinal axis. The length of each vessel is much greater than the diameter. In prior art liquid transfer systems, these vessels are typically disposed in an upright, or vertical, position wherein the length is disposed vertically. The main reason for disposing the vessels in an upright position is because the upright disposition allows the use of the capacitive type level sensors which provide sufficiently accurate measurements of liquid levels in the vessels while the vessels are disposed in an upright, or vertical, position.
It is desirable to be able to place the vessels in a horizontal position, wherein the length of the vessel is disposed vertically, in order to save space. However, capacitive-type level sensors are typically not as accurate in determining the amount of fluid stored in a vessel that is disposed in a horizontal position. This is due to the relatively smaller distance between the liquid level top surface and interior walls of the vessel opposite the liquid.
Further problems are incurred in determining the amount of fluid stored in a pressurizable vessel while the vessel is disposed in a horizontal position. Mechanical shock and vibration caused by the liquid transfer system and end use processing system create disturbances in the fluid stored in the vessels.
Accordingly, what is needed is an improved liquid transfer system using pressurizable vessels wherein the vessels are disposed in a horizontal position
What is also needed is an improved liquid transfer system using pressurizable vessels wherein the vessels are isolated from mechanical shock and vibration caused by the processing system.
Further needed is an improved liquid transfer system using pressurizable vessels, the system providing a sensor coupled with the vessel, and operative to generate a signal indicative of the amount of fluid stored in the vessel while the vessel is disposed in a horizontal position.